The present invention relates to a method for determining the range of vision in the field of view in front of a vehicle, the contrast of at least one object located in the field of view being measured using video technology.
German Published Patent Application No. 195 30 289 describes a plurality of methods for determining range of vision that are based on measuring contrast, absorption and reflection. The document states that contrast can be measured using video technology in order to determine the range of vision, but that this is not suitable for mobile applications in vehicles. Instead, the document proposes that range of vision be measured using the reflection principle, which is based on the backscattering of emitted light off water droplets, e.g., fog, that reduce visibility. The system then sends a signal to the driver if he has not brought the vehicle""s speed in line with the range of vision.
Systems for adaptive speed and headway control already exist and are known as ACC (Adaptive Cruise Control) systems. These ACC systems measure the distance between the vehicle and objects located in the area surrounding the vehiclexe2x80x94as a general rule this means other vehiclesxe2x80x94using radar or a laser (lidar). Herein, the field of view in front of the vehicle is scanned using a radar or laser beam, and the relative distance and, if necessary, the relative speed relative to the driver""s own vehicle is determined from the beams reflected off objects. The information regarding the relative distance and, respectively, relative speed supplied by the ACC system is also used to carry out speed control. ACC systems based on radar or lidar can detect objects in the vehicle""s field of view and measure the relative distance even if visibility is reduced, e.g., due to fog, snow or smoke. Because of this capability, the driver may rely on the ACC system and drive very fast even if visibility is significantly reduced. However, compared with a human being, a technical speed control system such as an ACC system can only detect the traffic situation in a very limited manner. It is therefore important that it be possible to deactivate the ACC system in certain traffic situations that the ACC system cannot handle, so that the driver himself controls the vehicle""s speed. Particularly in situations where the range of vision in the area surrounding the vehicle is very limited, one cannot leave the task of headway and speed control to the ACC system alone, because the human driver might not recognize soon enough that he is in a traffic situation that the ACC system cannot handle and thus might not intervene in the speed control system.
An object of the present invention therefore is to provide a method that allows the range of vision in the vehicle""s field of view to be determined as precisely as possible, so that with the help of the range of vision information the driver can be advised to modify his driving style.
According to the method according to the present invention, a monocular video sensor measures the contrast of the object detected by the radar or lidar sensor, and the distance to at least one object in the vehicle""s field of view measured by the radar or lidar sensor and the contrast of the object in question measured by the monocular video sensor are used to determine the range of vision.
According to another exemplary embodiment of the present invention, a binocular video sensor measures the distance to and contrast of at least one object, and the range of vision is determined from the measured contrast and measured distance values. In both methods, a distance measurement and a contrast measurement are taken, which means the range of vision in the vehicle""s field of view can be determined with great precision. In both cases, the contrast is measured using a passive video sensor system. Here, passive means that the field of view in which measurements are taken is not lit using an own light source, but rather only the light emitted by the field of view itself is recorded by the video sensor system.
Thus the distance measurement can be taken by the binocular video sensor as well as by a radar or lidar sensor. The advantage of this is that structures such as lane markings that can only be detected using a video sensor may be used to determine the range of vision as well as structures detectable by the radar sensor or lidar sensor. Aside from this, the advantage of using a video sensor is that it can be used not just to determine the range of vision but also to gather further information regarding the vehicle""s immediate environment, e.g., the course of the lane, road signs and similar.
It is usefull if the measured distance to an object whose measured contrast is still adequately detectable by the human eye is considered the range of vision. The video sensor should be brought in line with the wavelength range that is detectable by the human eye.
Preferably, the contrast measurement carried out by the video sensor is oriented to an object whose distance relative to the vehicle is changing. The contrast measurement and the distance measurement are then carried out iteratively.
It is useful to calculate the mean of the measured distances to different objects whose contrast constitutes a minimum value still adequately detectable by the human eye, which then indicates the range of vision. Alternatively, the range of vision is considered the greatest distance among the distances to different objects whose contrast constitutes a minimum value still detectable by the human eye. One can achieve a more precise result from measurements if each contrast measurement is carried out a plurality of times and a mean contrast value is determined from all the measurements.
To achieve a higher resolution in contrast measurement, it is useful to use a video sensor that has a non-linear transducer characteristic curve, the gradient of the characteristic curve decreasing as the light intensity increases.
It is advantageous if a headway control device in the vehicle is deactivated if the range of vision that has been determined falls below a minimum value.
The vehicle""s maximum desired speed may also be controlled as a function of the current range of vision that has been determined. It is useful to indicate the current range of vision to the driver inside the vehicle, or to send a signal to the driver indicating that the headway control device has been deactivated, or to send a signal to the driver indicating that the maximum desired speed that he has predefined has been reduced based on the current range of vision. All this information is used to advise the driver to modify his driving style if the range of vision is limited.